Drilling for oil or gas is a complicated and dangerous endeavor where drillers must blend both technology and experience to create a working well. The early stages of the well development are the most critical and due to the lack of blow-out protection, it is also the most hazardous period for the crew working the rig.
Setup for drilling an oil or gas borehole will vary depending upon location and ground conditions. However, in most circumstances, the first step is to install a relatively short string of large diameter conductor pipe. The conductor pipe will act as a drilling guide and prevents unconsolidated surface material from collapsing into the borehole. The pipe may be 120 feet long for an on surface development or a much longer string in an underwater drilling. In other applications, where the surface material has allowable cohesive properties, the operator will first drill an oversized hole using an auger prior to inserting the conductor pipe into the hole and then cementing the space between the outside of the conductor pipe wall and original hole. Once the conductor pipe is in place, the operator will prepare and level the pipe end by cutting and/or grinding. Additional sections of pipe will be welded onto the conductor to properly position the top flange with the drilling platform and to accept a top-drive head. A drilling fluid flow line attached to the conductor string must also be properly aligned to allow connection with a corresponding system on the drill platform. Leveling and positioning the top of the conductor pipe is call “nippling up”; the process is time consuming, typically requiring 12 to 14 hours.
The next step is to “drill for surface” or drill a smaller hole through the conductor pipe, preparatory for setting the borehole casing. The depth of the surface hole will vary depending on the depth of the ultimate target, ground conditions and other factors. However, this step may be the most perilous; and the shallower the bore, the greater the danger to the men and equipment. During the drilling process it is common for the drill hole to intercept gas pockets of methane or hydrogen sulfide in the rock or the coal formation being penetrated, the gases can be under extremely high pressure and are flammable. Drilling fluid or drilling “mud” is a mixture of water, clay and myriad other ingredients; the mud is injected under pressure down the center of the drill string, and as the mud recirculates back to the surface, on the outside of the drill string, the mud carries the rock cuttings out of the hole and serves to condition and seal the walls of the bore. Additionally, a driller may adjust the specific density of the drilling mud to increase hydrostatic pressure and help to control the flow of formation gas to the surface. However, if gas is encountered at shallow depth and the hydrostatic pressure of the drilling fluid in the vertical bore column is insufficient to overcome the pressure of the formation gas, the gas will blowout the top of the borehole at high speed, potentially causing equipment damage and injury, and if the gas is flammable, the danger of an explosion and fire is also present.
Once “surface” has been set, a casing string will be inserted, through the conductor, into the borehole and cemented into place. The cement will fill the void between the casing and the borehole and the space between the casing and the conductor. The casing can then be fitted with a BOP or blowout preventer. The BOP, as commonly known in the art, may be a ram or shear arrangement that blocks the well bore in the case of escaping gas or fluid. However, a BOP will not protect the drilling platform and crew until the casing is installed and they cannot protect the crew in the situation of a “behind casing blowout”.
A behind casing blowout occurs when gas escapes towards the drill platform between the casing and conductor. This may occur because the pressure wave from the escaping gas shocks the casing wall, fracturing or separating the cement between the casing and the conductor. It is also postulated that the differential of thermal expansion between the steel and cement creates separations or channels where high pressure gas can pass through.
What is needed, is a device that can be quickly installed, that will protect the drill platform and crew during the early startup stages, and can protect against a behind the casing blowout.